Disposable air filters are commonly used in small forced air handling units, such as residential heating, ventilating and air conditioning (HVAC) systems, and in portable air purification units. Such filters often include a frame and a fibrous filter material, and may include a reinforcing structure to help support the filter material. After a period of use, these filters become dirty or clogged and must be replaced. Proper filter maintenance helps keep the air handling unit operating at maximum efficiency and reduces operating costs.
Filter replacement is often done on a regular, fixed-interval basis. That is, the filter is replaced after a certain recommended fixed calendar period of time, such as three months, has passed. This fixed period of time, however, may not be appropriate for all situations (in particular, depending on the season of the year, which may affect how often the unit is actually operating during this calendar period). For example, the fixed time period may be too short, in which case the filter is discarded prematurely, or the fixed time period may be too long, in which case the filter is used beyond the time when it should have been changed.
Various filter change sensor systems have been proposed for indicating the need to change a filter. However, such systems tend to be complicated, may require one or more sensors to be placed within the air handling system (e.g., to measure pressure drop across the filter), and may require such sensors, controllers, etc. to be hard wired into the system. All of these can add to the cost and complexity of the system.